1. Field of the Invention
The present invention relates to organic materials for white-light-emitting and an organic electroluminescent device including a light-emitting layer made of the materials. The organic/polymer white-emitting devices are easy to form a film from a solution by spin casting and emit light under electric field. Thus, the device fabricated by using the materials can be applied to the back light of liquid crystal displays, lighting devices, or color flat panel displays.
2. Description of the Background Art
A white electroluminescent device using low molecular weight materials or polymer materials has been fabricated by mainly two methods. First, there is a method of manufacturing a multilayered device composed of a number of materials which emit lights of R, G and B colors, as carried out by Kido et al. [J. Kido, M. Kimura, K. Nagai, Science, 267, p 1332 (1995)], Xie et al. [Z. Y. Xie, Y. Liu, J. S. Huang, Y. Wang, C. N. Li, S. Y. Liu, J. C. Chen, Synth. Met. 106, p 71 (1999)], Ogura et al. [T. Ogura, T. Yamashita, M. Yoshida, K. Emoto, S. Nakajima, U.S. Pat. No. 5,283,132], and Deshpande et al. [R. S. Deshpande, V. Bulovic, S. R. Forrest, Appl. Phys. Lett. 75, p 888 (1999)]. In this method, it is difficult to form a multi-layered thin film and a white-emission can be obtained by trial and error to determine the thickness of each layer. In addition, disadvantageously the emission color may change depending on the applied voltage.
The other method is doping or blending organic luminescence colorant in luminescent host substances, as carried out by Granström et al. [M. Granström, O. Inganäs, Appl. Phys. Lett. 68, p 147. (1996)], Kido et al. [J. Kido, H. Shionoya, K. Nagai, Appl. Phys. Lett. 67, 2281 (1995)], Shi et al. [J. Shi, C. W. Tang, U.S. Pat. No. 5,683,823], and Chen et al. [S.-A. Chen, E.-C. Chang, K.-R. Chuang, U.S. Pat. No. 6,127,693]. Although the process in this method is simpler than the first method, a white-emission can be obtained by repeating trial and errors without a governing principle. Particularly, when blending or doping components have excellent miscibility between them, due to energy transfer from high-bandgap components to low-bandgap components, the spectrum of the host material may be largely varied depending on blending or doping level. Thus, it is difficult to predict the final emission spectrum. Particularly, when three or more components are blended to prepare a white-light-emitting material, it is more difficult to control energy transfer between the components. Successful white-light-emission depends on how energy transfer between the components to be blended is efficiently controlled.